1. Field of the Invention
This invention relates to a light-emitting diode and a method for making the same, more particularly to a light-emitting diode having a relatively high light extraction efficiency and a method for making the same.
2. Description of the Related Art
Referring to FIG. 1, a conventional light-emitting diode (LED) includes an epitaxial substrate 11, a light-emitting unit 12 and an electrode unit 13. The light-emitting unit 12 includes a first-type semiconductor layer 121 formed on the epitaxial substrate 11, a light-emitting layer 122 formed on a portion of the first-type semiconductor layer 121, and a second-type semiconductor layer 123 formed on the light-emitting layer 122. The electrode unit 13 includes a first electrode 131 electrically connected to the first-type semiconductor layer 121, and a second electrode 132 electrically connected to the second-type semiconductor layer 123. When electricity is supplied to the light-emitting unit 12 through the first and second electrodes 131, 132, the light-emitting unit 12 will emit light by virtue of the photovoltaic effect.
In general, the photoelectric efficiency of an LED is represented by an external quantum efficiency of the LED, which is a product of an internal quantum efficiency and light extraction efficiency of the LED. The light extraction efficiency means a ratio of the photon amount departing from the LED to the photon amount generated by the light-emitting layer 122. However, a GaN-based semiconductor material, which is a commonly used material for the light-emitting layer 122, has a refractive index of about 2.5. The refractive index of air is 1. Accordingly, when the light emitted from the light-emitting layer 122 travels to an interface between the light-emitting unit 12 and air, most of the light will be totally reflected and may not be emitted outwardly. Thus, the actual light extraction efficiency of the conventional LED is about 4% which is insufficient for photoelectric efficiency.
In order to enhance the light extraction efficiency of the LED, the epitaxial substrate 11 is normally etched to have a regular or irregular rough structure. Alternatively, a structure of a light-emitting surface of the light-emitting unit 12 may be varied to reduce the ratio of the total reflection at the interface between the light-emitting unit 12 and air, thereby enhancing the light extraction efficiency of the LED. However, when roughening the light-emitting surface of the light-emitting unit 12 using an etching process, the electrical properties of the LED may be adversely affected. In the case of patterning the epitaxial substrate 11 to have the rough structure, the epitaxial substrate 11 for the LED is commonly made of silicon, silicon carbide, aluminum oxide, etc., and is difficult to be etched. Accordingly, the etching process for forming the rough structure is time-consuming. Besides, the rough structure may have poor dimensional accuracy, thereby resulting in uneven luminance of the LED.